Geothermal Resources of the Yonezawa District in Yamagata Prefecture, Northeast Japan

Abstract

The Yonezawa district, located in the northeast Japan, has many spring areas. The area is underlain by pre-Tertiary basement rocks, Tertiary formations, Pliocene to Pleistocene deposits, and Quaternary volcanoes. Geothermal features in this area are clustered into two groups whether related to quaternary volcanoes. Geothermal resources related to quaternary volcanoes such as Azuma is accompanied by high temperature hot springs and hydrothermal alteration zones on the surface at present. Azuma volcano is distributed within the up rift zone and volcanic rocks erupted from it are mainly andesitic. The Ubayu depression is also located within the up lift zone and filled with acidic welded tuff. The Ubayu hot spring, situated near the Ubayu depression, is associated with an acidic alteration zone characterized by kaolinite, alunite and pyrophyllite. In Azuma area, maximum temperatures of the exploration wells are above 260 oC. The other side many non-volcanic hot springs are scattered mainly along the Yonezawa Basin. The high temperature hot springs Akayu is situated on margin of the Yonezawa Basin and in volcano-tectonic depressions, which is filled with thick submarine acidic pyroclastic flow. The Akayu depression, corresponding to low gravity anomaly, characterizes the geomorphology of this area and has a suitable structure favorable for thermal waters.